1. Field of the Invention
This invention relates to circular waveguides in general and more particularly to circular waveguides which are over-sized to provide a low-loss transmission path for microwave signals in the dominant TE.sub.11 mode.
2. Description of the Prior Art
It is known in the prior art to use over-sized circular waveguides to provide a low-loss transmission path for microwave signals in the dominant TE.sub.11 mode. For any mode of transmission of a microwave signal in a circular waveguide, the electric and magnetic transverse fields may each be resolved into a respective set of tangential and radial components. Those skilled in the art of microwave theory are aware that both the tangential and radial components vary periodically in amplitude along a circular path which is concentric with the wall of the waveguide, and also vary in amplitude along any given radius in a manner related to a Bessel function of order m. Modes of a transverse electric field are identified by the notation TE.sub.mn and modes of a transverse magnetic field are identified by the notation TM.sub.mn, where m represents the total number of full period variations of either the tangential or radial component of the respective electric or magnetic field along a circular path concentric with the wall of the waveguide, and n represents one more than the total number of reversals of polarity (sign) of either the tangential or the radial component of the respective electric or magnetic field along a radial path.
The dominant mode in circular waveguides is denoted as the TE.sub.11 mode, which corresponds to the TE.sub.10 mode in rectangular waveguides. It is well known in the prior art that the larger the cross-sectional area of a circular waveguide, or the higher the operating frequency, the greater will be the number of modes which may be supported within a circular waveguide. It is also known that it is desirable to confine the energy propagated in a circular waveguide to the dominant mode.
Higher-order mode signals may be generated and trapped by the terminations at the ends of an oversized circular waveguide, where the terminations form transitions to rectangular, single-moded waveguides. A radar system provides but one example of a microwave transmission system in which the above-referenced arrangement of circular and rectangular waveguides might be used. Higher-order mode signals which are spuriously generated and trapped between the transitions may be reflected back and forth in the circular waveguide before being dissipated. The reflected signals may produce false radar targets or echoes in the receiving apparatus, which are both undesirable and which degrade the performance of the radar system.
It is well known in the art to use rectangular waveguides which operate in the dominant TE.sub.10 mode to couple each end of a circular overmoded waveguide to other microwave components. A transition section is employed at each interface between the circular and rectangular waveguide to launch and receive the microwave signal, which is preferably transmitted in the circular waveguide in the TE.sub.11 mode.
One problem with over-sized circular waveguides operating in the dominant TE.sub.11 mode which are used in systems containing rectangular waveguides is that the circular waveguides can support a variety of higher-order modes, in addition to the desired TE.sub.11 mode.
Another problem associated with over-sized circular waveguides used in systems containing rectangular waveguides is that the circular waveguides can propagate higher-order modes which resonate between transitions which connect the circular waveguides to the rectangular waveguides. If the length of the circular waveguide is an integral number of half wavelengths at the chosen operating frequency, the resonance condition will degrade the transmission efficiency of the system by forming an attenuation peak which is produced by the higher-order mode energy trapped in the circular waveguide system being reflected at each end of the circular waveguide by the transition sections joining the circular waveguide to the rectangular waveguide sections.
Known prior art patents of interest which show forms of mode suppressors or absorbers include U.S. Pat. No. 3,218,586 issued Nov. 16, 1965; U.S. Pat. No. 3,016,502 issued Jan. 9, 1962; and U.S. Pat. No. 3,031,661 issued Apr. 24, 1962.